base.html

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarText" aria-controls="navbarText" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarText">
    <ul class="navbar-nav mr-auto">
      {% if user.is_authenticated %}
      
      <li class="nav-item">
        <a class="nav-link" href="/logout">Logout</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Welcome, {{user.username}}</a>
      </li>

      {% else %}

      <li class="nav-item">
        <a class="nav-link" href="/login">Login</a>
      </li>

      {% endif %}
    </ul>
  </div>
</nav>